1. Field of the Invention
This invention relates to a magneto-optical layer that provides high Faraday rotation and controlled coercivity.
2. Description of the Prior Art
When certain materials are subjected to a strong magnetic field, they become optically active. Thus, when plane polarized light is sent through such a material in a direction parallel to an applied magnetic field, the plane of polarization is rotated. This magneto-optical effect, called the "Faraday effect," has been observed in many solids, liquids, and gases and has found a variety of applications. In material having a magnetic moment, the amount of rotation of the plane of polarization is proportional to the magnetization and to the distance traveled through the medium. For a typical application, it is desirable that the constant of the proportionality, or "specific Faraday rotation," and transmittance be high.
One application, a magneto-optical light-switching array, dubbed "Lisa," was described by J. Gosch, Electronics, Dec. 29, 1981, pp. 53, 54. Potential applications for Lisa-type devices are in printing, electrophotography, and displays.
M. F. Shone et al., IEEE Trans. Magn. MAG-18, 1307 (1982), disclosed layers of the form Bi.sub.x Tm.sub.3-x Fe.sub.y Ga.sub.5-y O.sub.12, with x.gtoreq.0.6, y.apprxeq.1.2. The layers were grown by liquid phase epitaxy on gadolinium gallium garnet (GGG) substrates. These layers show high values of specific Faraday rotation (0.51-0.86 deg/.mu.m at 632 nm) and a broad range of anisotropy fields (1000-5000 Oe).
Nelson et al., J. Appl. Phys. 53, 1687 (1982), U.S. Pat. No. 4,295,988, issued Oct. 20, 1981, disclosed LPE magneto-optical crystals of Bi.sub.1 Lu.sub.2 Fe.sub.5 O.sub.12 garnet grown on GGG substrates. They showed that optical absorption of the layers could be reduced by the addition of CaO dopant. The LPE crystal is "constrained to have a similar lattice constant [to the GGG lattice constant]."
Mateika et al., U.S. Pat. No. 4,379,853, issued Apr. 12, 1983, disclosed a magneto-optical device whose substrate composition was chosen primarily to provide a match between the lattice constants of the magneto-optical crystal and substrate.
Bonner et al., U.S. Pat. No. 4,265,980, issued May 5, 1981, disclosed that LPE layers on GGG require that the lattice constant match within about 0.5% and that the substrate surface be smooth and flat, with a high degree of crystalline perfection.
Schmelzer et al., U.S. Pat. Nos. 4,274,935, issued June 23, 1981, and 4,314,894, issued Feb. 9, 1982, disclosed a method of preparing a magnetic layer that includes providing a lattice constant mismatch to cause the LPE layer to be stressed and thereby enhance ion implantation.
Breed et al., U.S. Pat. No. 4,435,484, issued Mar. 6, 1984, disclosed magnetic bubble devices consisting of monocrystalline nonmagnetic substrate bearing a magnetic iron garnet layer. The layer has a stress-induced uniaxial magnetic anisotropy component that results from a difference in lattice parameter between the layer and substrate. The mismatch may be as large as 1%, provided that no stress-relieving defects (cracks, tears, etc.) are generated. A similar disclosure was made earlier by Stacy et al., U.S. Pat. No. 4,169,189, issued Sept. 25, 1979.
Volluet et al., U.S. Pat. No. 4,316,162, issued Feb. 16, 1982, disclosed a magnetostatic wave device on a GGG substrate, in which part of the substrate is roughened to increase the attenuation of the magneto-static wave.
In general, the teaching of the prior art relating to magneto-optical layers emphasizes the desirability of a close match between the lattice constants of the layer and substrate crystals. Where a mismatch between the lattice constants has been disclosed, in each case the device was not magneto-optical.